This invention relates generally to the utilization of lower alkanes and the synthesis of hydrocarbons therefrom and, more specifically, to conversion of a low molecular weight alkane, such as methane, to a higher molecular weight hydrocarbon.
As the uncertain nature of ready supplies and access to crude oil has become increasingly apparent, alternative sources of hydrocarbons and fuel have been sought out and explored. The conversion of low molecular weight alkanes (lower alkanes) to higher molecular weight hydrocarbons has received increasing consideration as such low molecular weight alkanes are generally available from readily secured and reliable resources. Natural gas, partially as a result of its comparative abundance, has received a large measure of the attention focused on sources of low molecular weight alkanes. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during numerous mining operations, in various petroleum processes, and in the above- or below-ground gasification or liquefaction of coal, tar sands, oil shale and biomass, for example. Generally, however, much of the readily accessible natural gas has a high valued use as a fuel whether in residential, commercial or industrial applications.
Additional major natural gas resources, however, are prevalent in many remote portions of the world such as remote areas of Western Canada, Australia, U.S.S.R. and Asia. Commonly, natural gas from these types of resources is referred to as "remote gas." Of course accessibility is a major obstacle to effective and extensive use of remote gas. Consequently, methods for converting low molecular weight alkanes, such as those present in remote gas, to higher molecular weight hydrocarbons, preferably, to more easily transportable liquid fuels, are desired and a number of such methods have been reported.
For example, G. E. Keller and M. M. Bhasin (J. Catal., 73, 1982, 9-19) have shown that methane can be converted to C.sub.2 hydrocarbons in the presence of reducible metal oxide catalysts but that the yields of ethylene and ethane are low and amount to only from 10 to 50 percent of the reacted methane. To improve the selectivity for the production of the desired C.sub.2 hydrocarbons and to suppress the undesirable further reaction of the C.sub.2 hydrocarbons initially formed to carbon dioxide, Keller and Bhasin proposed a special reaction method generally involving a sequence of four steps;
(1) charging the catalyst with oxygen by passing an oxygen-containing gas over the catalyst;
(2) replacing the oxygen in the gas chamber of the catalytic reactor with an inert gas;
(3) feeding methane over the catalyst, which partially produces the desired reaction; and
(4) supplanting the residual methane and resulting product in the reactor with an inert gas before the sequence of steps is repeated.
In this process, depending on the catalyst used and the temperature selected, the selectivities for the production of C.sub.2 hydrocarbons range from about 5% to about 45%, the selectivities for the production of CO.sub.2 range from about 55% to 95%, and the conversions of methane range between 1% and 10%.
Keller and Bhasin arrived at the conclusion that oxidative coupling is only highly selective to higher hydrocarbons when the coupling reaction takes place in the absence of gas-phase oxygen and that the oxidative coupling of the hydrocarbons should be caused by reaction with the lattice oxygen of the metal oxide "catalyst," resulting in the reduction of the valence level of the metal oxide. [NOTE: The term "catalyst" as used herein does not have its standard meaning as while a relatively small amount of the specified material notably affects the rate of the chemical reaction, the material itself or at least a component thereof is consumed or undergoes a chemical reaction.] Thus, since the catalyst has only a predetermined amount of lattice oxygen available, only a limited quantity of hydrocarbon can be reacted for every measured unit of catalyst before the catalyst needs to be regenerated, e.g., with oxygen being taken up by lattice openings.
It is evident that the modus operandi in Keller and Bhasin is costly in terms of apparatus as well as simultaneously being linked with relatively smaller yields in space-time terms and high operating and investment costs. Moreover, according to the data of the authors, the attainable methane conversions and/or the resultant spacetime yields are generally believed to be too small for commercial installations. Furthermore, the only products reported are C.sub.2 hydrocarbons.
Subsequent to the publication of the findings of Keller and Bhasin, the efforts of a number of other researchers in the area of oxidative coupling have been reported, published and/or patented. For example, Jones et al., U.S. Pat. Nos. 4,443,664-9 disclose methods for synthesizing hydrocarbons containing as many as 7 carbon atoms from a methane source which comprise contacting methane with a reducible oxide of antimony, germanium, bismuth, lead, indium or manganese. These patents also disclose that the reducible oxides can be supported by a conventional support material such as silica, alumina, titania and zirconia. The ranges of reaction temperatures disclosed in the aforesaid patents are from a lower limit 500.degree. C. to an upper limit of 800.degree. C.-1000.degree. C. In the disclosed processes (hereinafter referred to as a "redox" process or mode of operation), the reducible oxide is first reduced and then regenerated by oxidizing the reduced composition with molecular oxygen, either in a second zone or by alternating the flow of the feed gas, e.g., a methane-containing gas, with the flow of an oxygen-containing gas. The highest yield of hydrocarbon products reported was only about 2.1% of the methane feed, when a reducible oxide of manganese was employed.
Baerns, West German Patent Application No. 3,237,079.2, discloses a method for the production of ethane or ethylene by the reaction of methane and oxygen-containing gas at a temperature between 500.degree. C. and 900.degree. C., at an oxygen partial pressure of less than about 0.5 atmosphere at the reactor entrance, with a ratio of methane partial pressure to oxygen partial pressure greater than 1 at the reactor entrance and in the presence of a solid catalyst free of acidic properties. As disclosed, the method can be performed with or without recycle of remaining unreacted methane. The highest molecular weight product formed in the disclosed method is propane and the highest collective selectivity for the formation of ethane, ethylene and propane is only about 65% of methane converted.
Baerns discloses that oxides of metals of Groups III and VII of the Periodic Table are suitable for use as catalysts in the methods disclosed therein and that the oxides of lead, manganese, antimony, tin, bismuth, thallium, cadmium and indium are particularly preferred. Baerns further discloses that the metal oxides can be employed with or without a carrier and that specifically preferred carriers are alumina, silica, silica carbide and titania. Specific examples of carrier materials disclosed were formed from gamma-alumina having BET surface areas of 160-166 m.sup.2 /gm, silica having a BET surface area of 290 m.sup.2 /gm, bismuth oxide, alumina silicate and titania.
Ito et al., "Synthesis Of Ethylene and Ethane By Partial Oxidation of Methane Over Lithium-Doped Magnesium Oxide," Nature, Vol. 314, (April 25, 1985) 721-722, discusses the use of lithium-doped magnesium oxide in the partial oxidation of methane to more useful chemicals such as higher molecular weight hydrocarbons including methanol, ethylene, and benzene. Therein, a yield of 19% for C.sub.2 compounds with 50.3% selectivity at 37.8% conversion was reported and noted to be considerably better than the results reported in the literature for other metal oxides.
Ito et al., "Oxidative Dimerization Of Methane Over A Lithium-Promoted Magnesium Oxide Catalyst," J. Am. Chem. Soc., Vol. 107, (1985) 5062-5068, discusses the use of lithium-promoted magnesium oxide in the conversion of methane to higher molecular weight hydrocarbons such as ethane and ethylene, for example. Therein, C.sub.2 compounds obtained with 50% selectivity at a 38% conversion of CH.sub.4 over 4 grams of a catalyst having 7 wt. % lithium-promoted magnesium oxide at 720.degree. C. were reported. It is noted that better selectivities for C.sub.2 (ca. 70%) were achieved over catalysts promoted with 7% or more Li.sup.+. The article advances a model for the selective conversion of CH.sub.4 wherein the major part of the activity results from the substitution of a monovalent cation into a divalent site and thus calls for the alkali metal ion to be substitutable for the alkaline earth ion.
International Publication WO 86/05176 broadly discloses a method for converting a feedstock alkane containing from one to three carbon atoms to higher molecular weight hydrocarbons. The method includes the steps of contacting the feedstock alkane with an oxygen-containing gas in a reactor in a presence of an oxidative coupling catalyst to produce a gaseous mixture including higher molecular weight saturated and unsaturated aliphatic hydrocarbon products followed by contacting the resulting gaseous mixture with an oligomerization catalyst. Oxidative coupling catalysts disclosed to be useful in the practice in the invention include silica having a surface area of less than about 175 m.sup.2 /gm and reducible compounds of lead, antimony, germanium, vanadium, tin, bismuth, cadmium, indium, manganese, thallium or mixtures thereof.
The publication, 8th International Congress on Catalysis, in an article entitled, "Oxidative Dehydrogenation and Coupling of Methane," Hinsen et al., pp. 581-593 (July, 1984) describes a process for the oxidative coupling of methane utilizing a PbO supported catalyst. Various supports were tested, including: .gamma. and .alpha.-Al.sub.2 O.sub.3, TiO.sub.2, alumina silicate and silica gel. These support materials had very high surface areas, ranging, for example, from 110 m.sup.2 /g for .gamma.-Al.sub.2 O.sub.3 up to 245 m.sup.2 /g for the SiO.sub.2 gel with the catalyst typically being prepared by incipient wetness techniques whereby the pores in the support material are filled with liquid. The use of such high surface area materials in catalysts to be used for oxidative coupling is generally undesirable as the resulting high surface area catalysts typically have relatively high selectivity to "combustion products," such as CO and CO.sub.2. In oxidative coupling, the formation of such combustion products is generally sought to be minimized as the formation of such carbon-containing combustion products is typically viewed as being at the expense of reduced production of desired higher molecular weight hydrocarbons.
Over the years various additional oxidative coupling catalyst, contact agent, contact solid or the like compositions; additives, promoters, or the like for addition thereto and processes for oxidative coupling have been tested, reported or disclosed with varying degrees of success. Typifying these materials are those found in U.S. Pat. Nos. 4,444,984; 4,533,780; 4,547,607; 4,554,395; 4,567,307; and 4,568,785.
More particularly, U.S. Pat. Nos. 4,489,215; 4,495,374; 4,499,322; 4,499,323; 4,499,324; 4,450,310; 4,523,049; 4,656,155; 4,721,828; and 4,727,212; while disclosing processes utilizing compositions which include an alkali metal, an alkaline earth metal or combinations thereof, require, as a key component, a reducible metal oxide and/or depend on the utilization of lattice oxygen.
For example, U.S. Pat. No. 4,450,310 discloses a methane conversion process for the production of olefins and hydrogen wherein methane is contacted with a catalyst including the mixed oxides of a Group IA metal, a Group IIA metal and optionally a promoter metal, such as copper, rhenium, tungsten, zirconium, rhodium and mixtures thereof, in the absence of oxygen and water, to produce olefins and hydrogen. Thus, the contacting of methane with the catalyst material is done in the absence of oxygen.
U.S. Pat. No. 4,523,049 discloses a method for converting methane to higher hydrocarbon products wherein a methane-containing hydrocarbon gas and an oxygen-containing gas are contacted with a reducible metal oxide under synthesis conditions. The contact solids used therein are disclosed as using a promoting amount of alkali metal, alkaline earth metal and/or compounds thereof. Thus, this patent also requires the presence of a reducible metal oxide, which reducible metal oxide may require periodic reoxidation to maintain the usefulness or efficiency of the contact solid for the conversion of methane.